In the future https://www.w3.org/TR/webcodecs/ may allow for a native API to do this.
It may make some things this can do possible but ffmpeg has support for so many codecs and so many operations and is incredibly extensible that I doubt anything short of making ffmpeg available in the browser will give you that much power.
In the future https://www.w3.org/TR/webcodecs/ may allow for a native API to do this.